Lymphocyte blastogenesis to various T. cruzi antigens was tested in patients with chronic Chagas' disease and found to be present but variable in degree. Magnitude of response of this test of cell-mediated immunity showed no obvious correlation with clinical features of disease. During the same study the frequency of HLA tissue types in Chagas' patients was compared to appropriate controls. A reduced frequency on one antigen type at the A locus and possible increased frequency of two antigenic types, one at the A and the other at the B locus, was found in patients with chronic Chagas' disease. A plaquing technique for quantitative estimation of minimally infective units of T. cruzi for cells has been developed. This technique appear applicable to testing strains of T. cruzi directly from infected bugs. The genetic pattern of susceptibility to a strain of Leishmania tropica in mice suggests control by a single gene.